This invention relates to a pincushion correction circuit which is useful in a television apparatus.
A side or East-West pincushion correction apparatus is described in U.S. Pat. No. 4,088,931 issued May 9, 1978 in the name of Peter E. Haferl entitled, "PINCUSHION CORRECTION CIRCUIT", in which an impedance network is coupled in series with the horizontal deflection coil. A switch is coupled in parallel with the impedance. The switch is operated during the second half of the horizontal retrace interval at times which are progressively advanced and retarded during each cycle of the vertical deflection. The average impedance in series with the horizontal deflection coil is thereby varied at a vertical deflection rate in such a manner as to correct for horizontal or side pincushion distortion. A damping circuit consisting of a resistor is coupled between a point on the impedance network and ground in order to minimize the effect of switching transients and thereby prevent undesirable oscillations in the switched impedance. The resistor may dissipate substantial amounts of power.
In the aforementioned arrangement, pincushion correction is achieved by controlling the amount of energy or current in the deflection winding at the beginning of the trace interval. This in turn is accomplished by controlling the impedance in series with the deflection winding during the second half of the retrace interval. When the average impedance is high, less current tends to flow in the deflection winding and the horizontal trace width is reduced. When the average impedance is low, more current flows in the deflection winding and the trace width increases. The average impedance in series with the deflection winding is controlled by the relative time at which the switch is closed. A smooth, continuously variable pincushion correction requires that the voltage waveform across the impedance network follow the waveform of the retrace pulse without transient voltages or undamped oscillations. Such transient voltages or oscillations cause the pincushion correction to vary in a manner not continuously related to the time at which the switch is closed.
The amount of power dissipated in the pincushion circuit damping resistor as described in the aforementioned Haferl application may be somewhat reduced by coupling the damping resistor directly across the switch. This has the advantage that horizontal deflection signals do not appear across the damping resistor except during the horizontal retrace interval, resulting in reduced power dissipation.
The power dissipation of the damping circuit can be almost completely eliminated by a second controllable switch coupling the pincushion distortion control impedance to a reference voltage source, as described in copending application Ser. No. 821,784 filed Aug. 4, 1977, in the name of Willem den Hollander. A particularly advantageous embodiment of this arrangement uses a diode as the controllable switch, and a sample of the retrace voltage from a deflection transformer as both the switch control voltage and the reference voltage source.
While the aforementioned den Hollander damping circuit has very low power dissipation, it may not be possible to use the diode switch arrangement in all applications, as where a well-defined switching voltage is not available from the deflection transformer, or where direct voltages associated with the transformer winding perturb diode switching, or where the horizontal output transformer also provides isolation of the receiver chassis from the alternating power lines. For those damping applications in which the switched damping circuit is not used, lower power dissipation than that provided by the Haferl arrangement may be desirable.